Local area networks (LANs) provide for the interconnection of a multiplicity of endstations so that a multiplicity of users may share information in an efficient and economical manner. Each endstation of a LAN can typically communicate with every other endstation that is physically connected to that LAN.
As the number of endstations of a LAN increases, the effective throughput for each endstation of the LAN decreases. To increase the throughput for each endstation, the LAN can be "segmented" into smaller interconnected sub-networks or "LAN segments" that each have fewer endstations. The load for each sub-network of a segmented LAN is reduced, leading to increased throughput for a segmented LAN when compared to a similarly sized, unsegmented LAN.
Interconnection of the segments of prior segmented LANs is achieved by connecting several individual sub-networks to the ports of a "switching fabric circuit." The term "switching fabric circuit" as used here is meant to encompass any circuit that provides for the processing and forwarding of information between LAN segments in a segmented network. For example, one prior switching fabric circuit includes a number of conventional Ethernet bridges connected to a backbone network that is controlled by a system processor. The system processor is responsible for filtering and forwarding frames of data between LAN segments. Filtering and forwarding typically requires first storing incoming frames of data received from the ports and subsequently forwarding the frames of data to the appropriate destination port. One disadvantage of the filtering and forwarding schemes of prior bridging architectures is the delay time associated with the filtering and forwarding process.
To address this and other disadvantages of bridging architectures, a second prior switching fabric circuit commercially known as Etherswitch.RTM. was developed. Etherswitch.RTM. is sold by Kalpana, Inc., of Sunnyvale Calif., and described in U.S. Pat. No. 5,274,631, of Bhardwaj, issued on Dec. 28, 1993, entitled Computer Network Switching System. The Etherswitch.RTM. includes a number of packet processors, one for each port, that are each connected to multiplexor logic. The multiplexor logic acts as a crossbar switch that provides a direct physical connection between the packet processors of the source and destination ports. The multiplexor logic allows for packets to be transmitted directly from the packet processor of the source port to the packet processor of the destination port without first storing the packets. The process of forwarding a packet without first storing the packet is known as "switching on-the-fly."
Wherein on-the-fly switching provides significant speed advantages over other prior art switching fabric circuits, the use of the multiplexor logic effectively limits the number of ports of the switching fabric circuit. Because the multiplexor logic provides a physical connection between ports, the hardware of the multiplexor logic is designed to service a fixed number of ports, and providing additional ports to the multiplexor logic to allow for future expansion may be cost-prohibitive. For example, doubling the maximum number of ports typically results in squaring the complexity of the multiplexor logic, which greatly increases the cost of the multiplexor logic.
Further, each port is typically limited to only a portion of the total bandwidth for the switching fabric circuit by virtue of the physical link between the port and the multiplexor logic. For example, the total bandwidth for a ten port Etherswitch may be 100 Mb/s, wherein each port is provided 10 Mb/s of bandwidth. No port may request more than its 10 Mb/s of bandwidth. A port is therefore unable to use the unused bandwidth of idle ports to increase its individual throughput.